Optical density-calibrated photometer

ABSTRACT

A system in which changes in the optical density of a sample are measured by passing a light beam through the sample including a photomultiplier for intercepting the exit beam and providing an electrical signal corresponding to its intensity. An adjustable high voltage source is used to adjust the output voltage of the photomultiplier circuit to a predetermined reference level. An amplifier network and voltage divider having stepped positions calibrated to directly indicate changes in optical density per volt of output couples the voltage to a fixed gain (volt/cm) oscilloscope or recorder so that change in optical density is directly related to the displacement of the oscilloscope or recorder trace.

United States Patent Ernest S. Gordon 12176 Terrence, Saratoga, Calif.95070 211 Appl.No. 631,061

[22] Filed Apr. 14, 1967 [45] Patented May 25,1971

{-72} Inventor [54] OPTICAL DENSITY-CALIBRATED PHOTOMETER 8 Claims, 1Drawing Fig.

[52] U.S. Cl. 356/226,

250/207, 250/218 [51] Int. Cl G0lj 1/44 [50] Field of Search 356/96,

[5 6] References Cited UNITED STATES PATENTS 2,900,510 8/1959 Sparks,Jr. 250/43.5 3,097,563 7/1963 Weisglass 250/207X OTHER REFERENCESRuiter, Quantitative Measurements in Modern Oscilloscopes and TheirUses," Constable and London (London) 1957, pp 311-323 & Title Page andFly Leaf.

ZAK et al., Automation for Copper, Iron and Zinc in Mixtures, Microchem1., Vol. V1, 1962, pp 67- 71.

TEKTRONIX TYPE 532 CRO lNSTRUCTlON MANUAL (1960) pp. 3- l, 6- 1 thru 6-3, 2 Summary of Features" ages.

TEKTRONIX INSTRUCTION MANUAL FOR Dual- Trace Calibrated Preamp Type53/54 C, pp. l- 5 (EX- aminer s Designation) June 1958 PrimaryExaminer-Ronald L. Wibert Assistant Examiner-R. J. WebsterAttorneysRobert J. Steinmeyer and Paul R. Harder ABSTRACT: A system inwhich changes in the optical density of a sample are measured by passinga light beam through the sample including a photomultiplier forintercepting the exit beam and providing an electrical signalcorresponding to its intensity. An adjustable high voltage source isused to adjust the output voltage of the photomultiplier circuit to apredetermined reference level. An amplifier network and voltage dividerhaving stepped positions calibrated to directly indicate changes inoptical density per volt of output couples the voltage to a fixed gain(volt/cm) oscilloscope or recorder so that change in optical density isdirectly related to the displacement of the oscilloscope or recordertrace.

OPTICAL DENSITY-CALIBRATED PI-IOTOMETER BACKGROUND OF THE INVENTION Thisinvention relates to photometers and particularly to spectrophotometersused in the investigation of the kinetics of chemical reactions.Heretofore, measurements of changes in the optical density of samples ofmaterials have been investigated by photometric techniques and haverequired absolute conversion from photomultiplier output units intooptical density units to compute the difference. This leads totimeconsuming, cumbersome, and repeated calibrations of equipment andcalculations of experimental results. There is, therefore, a need for anew and improved photometer.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a photometer which will overcome the above-named limitations anddisadvantages by having an output directly calibrated in units ofoptical density change per volt of output.

Another object of the invention is to provide a photometer of the abovecharacter which is particularly adapted for studying the kinetics ofchemical reactions.

Another object of the invention is to provide a photometer of the abovecharacter which is simple to set up and calibrate and which provides arange of easily changed calibrated sensitivities.

Another object of the invention is to provide a photometer of the abovecharacter which eliminates the need for manual calculations to obtainoptical density change measurements.

In accordance with the above objects there is provided a photometricsystem for measuring changes in the optical density of a sample throughwhich a light beam is passed. Such a photometric system includes aphotoelectric transducer adapted to intercept the light beam passed bythe sample and 3 to provide an electrical signal having an amplitudecorresponding to the intensity of the light beam. The electrical signalis displayed on suitable display means so that its value can be followedin detail. The invention proposes that the display means be providedwith a predetermined gain in centimeters of deflection per volt ofinput. There isprovided a calibration network which includes means foradjusting the output of the photoelectric transducer to a predeterminedreference level and means responsive to changes in the output of thetransducer for supplying a voltage proportional to the change. Thelatter includes an attenuation means for controlling the value of thevoltage in accordance with the magnitude of the voltage change and forsupplying that output to the display means. The attenuation means iscalibrated to directly indicate change in optical density per volt ofoutput.

These and other objects and features of the invention will becomeapparent from the following description when taken in conjunction withthe accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENT The drawingschematically shows the electrical circuit for carrying out theinvention together with a diagrammatic illustration of the pertinentportions of a photometer in which it is incorporated.

Referring to the drawing, there is shown a photometric system formeasuring changes in the optical density of a sample including a lightsource 11 which may be monochromatic having an adjustable frequencyoutput as in the conventional spectrophotometer. The output of the lightsource llis passed through a sample cell holder 12 containing a samplecell 13 having transparent end windows for permitting the light energyto pass through the cell. The cell 13 serves to contain a sample 14 ofthe material being analyzed. For reaction kinetics studies the sample ofmaterial is typically rapidly injected at a predetermined time and thechanges in the transmission of the light beam through the sample areobserved to detennine the nature of the reaction kinetics taking place.Another common technique disturbs the chemical equilibrium of the sampleby joule heating so as to effect its light transmission characteristics.The output of light beam from the sample is intercepted by aphotoelectric transducer 16 which may be a photomultiplier tube ofconventional type having a cathode 17, anode 18 and dynodes19-1,417849l4 217914 3.2-952 19-2, 19-3. Transducer 16 provides anelectrical output signal on anode 18 which has an amplitudecorresponding to the intensity of the light beam impinging on thetransducer.

Means are provided for adjusting the output of the transducer to apredetermined reference level. For the photomultiplier tube illustratedin the drawing, such means includes a voltage divider network consistingof resistors R1, R2, R3 and R4, the nodes between portions thereof beingconnected to the dynodes 19-1, 19-2, 19-3 of the photomultiplier tube asshown. The resistance of resistor R1 is made adjustable so that theoutput of the photomultiplier can be increased or decreased.

The output of the photomultiplier is connected through an ammeter 21 toa load resistance consisting of resistor R5 connected in series with theadjustable input ofa balancing potentiometer R6, one end of which isconnected to common potential and the other end of which is connected toa positive bias voltage through a resistor R7. The resistor R5 and thatportion of R6 connected between the adjustable arm and the commonpotential form a voltage divider, the resistance of R6 being a minorportion of the total load resistance presented by the voltage divider.The voltage appearing on the input or microarnmeter side of resistanceR5 is coupled to a wide band DC pulse preamplifier through a couplingcapacitor C1. A bypass switch 23 is connected in parallel with couplingcapacitor Cl and cooperates with R6 to provide a DC-coupled operation ashereinafter explained.

Means are provided which are responsive to changes in the 5 outputvoltage of the photoelectric transducer appearing across resistor R5 andpotentiometer R6 for supplying a voltage proportional to changes in theoutput. Such means includes a DC pulse preamplifier 24 connected inseries with an attenuation means 26 and DC buffer amplifier 28 theoutput of which is supplied to display means such as oscilloscope 30 orchart recorder 32. The display means is set for a predetermined fixedgain in centimeters of deflection per volt of input for providing avisual display of electrical signals applied to thereto in real time.

The attenuation means 26 controls the value of the voltage incrementapplied to the display means and maintains it within its range. Suchmeans comprises a voltage divider having a plurality offixed'resistances R8, R9 and R10 connected in series and contact means34 for selectively connecting the output of the divider to one of thenodes between adjacent resistances. The values of resistances R8, R9 andR10 are selected such that each position of the contact corresponds to aprecalibrated value which directly indicates changes in optical densityper volt of output.

The following discussion provides the justification for the conversionfrom a voltage change to a calibration in terms of optical densitychange:

The definition of optical density (or absorbance) is:

where 1 light energy incident I light energy transmitted (throughsample) Let the initial 00. of the sample, say 0.0. be:

where I energy passed at time 1? and let The incremental O.D., sayA0.D., between times t,, t

Say that the increment of change of 0D. is small, so that (I2 I1)/I1 Itcan be shown by way of an expansion series that 1n(l+a) a where a l.

Therefore,

It can further be shown that for a AO.D. 0.03, the error from the aboveapproximation is small. Since photocurrent is directly proportional,'within design limits, to light energy received, and if the phototubeload resistance R5, R6 is constant, AO.D. can be expressed in terms ofload resistance voltage drop, v:

Referring again to the drawing, a predetermined value of V, is set byadjusting potentiometer R1 to control the photomultiplier dynodevoltages so that the voltage drop appearing across load resistance R5and R6 attains a predetermined reference level as indicated by thedeflection of microammeter 21. The amplified output voltage delivered tothe oscilloscope is controlled with the calibrated voltage divider 26.Resistance values R8, R9, R10 are chosen so that positions 1, 2, 3,etc., correspond to values of AO.D. per centimeter of oscilloscopedeflection (AO.D./cm.) such as, for example, 0.001, 0.002, 0.005AO.D./cm. The oscilloscope 30 sensitivity is fixed and remains at apredetermined fixed value. The chart recorder 32, if used, is initiallyset in sensitivity so that its full scale deflection corresponds to thatof the oscilloscope 30 so that the recorder 32 also will read directlyin AO.D. per centimeter of deflection.

For recording fast changes in optical density, and slower changes downto the order of 1 second, the switch 23 bypassing coupling capacitor C1is left open. For changes slower than this, switch 23 is closed to allowdirect coupling of the system. In this case the balancing potentiometerR6 is adjusted for zero baseline before each run. The incrementaloptical density per centimeter calibration is not appreciably effectedby this procedure provided that R6 is made sufficiently small withrespect to R5, say less than about one-hundreth the value thereof.

Summarizing the general operation, R] is first adjusted for a presentvalue, for example, a 40 microampere deflection on microammeter 21. Ifoperating DC coupled, R6 is set for zero output from amplifier 24. Thecalibrated voltage divider 26 is set for the anticipated change ofoptical density expected from the run. The change in optical density(A0.D.) obtained is then simply determined by multiplying the number ofcentimeters in deflection of the scope (or recorder) by the calibrationsetting of voltage divider 26 (OD. per centimeter).

To those skilled in the art to which the invention relates many changesin the procedures and widely differing embodiments and applications ofthe invention will suggest themselves without departing from its spiritand scope. For example, the calibration technique herein may be employedin any photometer or spectrophotometer system wherein change in opticaldensity is to be measured. Many such systems use simple phototubes orphotocells instead of photomultipliers and it will be obvious 'thatoptical apertures may be provided to produce the present value ofphotocell current for the calibration procedure.

lclaim:

1. In a system for measuring changes in the optical density of a samplethrough which a light beam of first intensity is transmitted under firstpredetermined conditions and through which a light beam of secondintensity is transmitted under second predetermined conditions such thatthe change in optical density is proportional to the ratio of the firstand second intensity difference to the first intensity, said systemincluding a photoelectric transducer adapted to intercept said lightbeam transmitted by the sample and to provide electrical signals havingamplitudes corresponding to the first and second intensities of thelight beam and display means having a predetermined gain in centimetersof deflection per volt of input for providing a visual display of saidelectrical signals, a calibration network comprising means for adjustingthe inagnitude of the output of said photoelectric transducer to apredetermined reference voltage level under the first predeterminedconditions, means responsive to changes in the output of saidphotoelectric transducer under the second predetermined conditions forsupplying a voltage change proportional to the change in the output,such that the change in optical density is proportional to the ratio ofthe voltage change to the reference voltage level, said last named meansincluding an attenuation means for controlling the value of themagnitude of the voltage change, according to the magnitude of thechanges in the output of said photoelectric transducer, the output ofsaid attenuation means being sensed by said display means, saidattenuation means being calibrated to indicate changes inoptical'density per unit of deflection of the display means.

2. A system as in claim 1 wherein said attenuator means is a voltagedivider.

3. Apparatus as in claim 1 wherein said means responsive to the outputof the photoelectric transducer includes an ammeter in series with aload'resistance, the gain of said photoelectric transducer beingadjustable to obtain a predetermined current through said loadresistance.

4. Apparatus as in claim 2 wherein said voltage divider comprises aplurality of fixed resistances connected in series and switching meansfor selectively connecting the output of the attenuator to one of thenodes between adjacent resistances.

5. Apparatus as in claim 2 wherein a wide band pulse amplifier isconnected in series in the load resistance and the voltage divider.

6. Apparatus as in claim 5 further including a DC-blocking capacitorconnected in series with the input of said amplifier so that saidamplifier is sensitive only to changes in the voltage developed acrosssaid load resistance.

7. Apparatus as in claim 5 further including a switch connected acrosssaid capacitor for selectively bypassing the same and wherein said loadresistance includes a voltage divider the resistance of which is a minorportion of the total resistance of said load resistance one side of saidpotentiometer being connected to a bias potential, the adjustable arm ofsaid potentiometer being connected to the other portion of said loadresistance, and the other fixed arm of said potentiometer beingconnected to reference potential adjustment of said potentiometerserving to offset the reference voltage level to establish a variablereference baseline when said switch is closed.

8. Apparatus as in claim 1 further including a buffer amplifierconnected in series between the output of said voltage divider and saiddisplay means.

2. A system as in claim 1 wherein said attenuator means is a voltage divider.
 3. Apparatus as in claim 1 wherein said means responsive to the output of the photoelectric transducer includes an ammeter in series with a load resistance, the gain of said photoelectric transducer being adjustable to obtain a predetermined current through said load resistance.
 4. Apparatus as in claim 2 wherein saId voltage divider comprises a plurality of fixed resistances connected in series and switching means for selectively connecting the output of the attenuator to one of the nodes between adjacent resistances.
 5. Apparatus as in claim 2 wherein a wide band pulse amplifier is connected in series in the load resistance and the voltage divider.
 6. Apparatus as in claim 5 further including a DC-blocking capacitor connected in series with the input of said amplifier so that said amplifier is sensitive only to changes in the voltage developed across said load resistance.
 7. Apparatus as in claim 5 further including a switch connected across said capacitor for selectively bypassing the same and wherein said load resistance includes a voltage divider the resistance of which is a minor portion of the total resistance of said load resistance one side of said potentiometer being connected to a bias potential, the adjustable arm of said potentiometer being connected to the other portion of said load resistance, and the other fixed arm of said potentiometer being connected to reference potential adjustment of said potentiometer serving to offset the reference voltage level to establish a variable reference baseline when said switch is closed.
 8. Apparatus as in claim 1 further including a buffer amplifier connected in series between the output of said voltage divider and said display means. 